Rod saver speed control method and apparatus

ABSTRACT

A motor speed controller ( 100 ) identifies the position of a pump rod ( 14 ). At one or more critical positions of the rod, the motor speed controller ( 100 ) adjusts the speed of the motor to change the movement of the pump rod ( 14 ). The critical positions and the rate of change of speed at each critical position can be user-specified.

CLAIM OF PRIORITY

This application claims the benefit under 35 U.S.C. 119(e) of U.S. Provisional Patent Application No. 60/319,022 filed on Dec. 3, 2001.

FIELD OF THE INVENTION

This invention relates to rod pump controls and, more particularly, to a method and apparatus for accurately changing the speed of movement of a rod pump during its repeating cycle.

DESCRIPTION OF THE PRIOR ART

Rod pumps have long been used to remove oil from wells. The subsurface pump is connected to a surface drive system or prime mover by a rod string. The rod string is a collection of rods that mechanically connect a pump mover to the subsurface pump. The rod string permits the subsurface pump to be lowered (downstroke) into a well bore and raised (upstroke) from the well bore.

During pump operation, polish rod and pull rods of the rod string are moved up and down. The rod string stretches during the upstroke and compresses during the downstroke. The up and down motions of the rod string are represented graphically as a sinusoid. Historically, the pump motor was operated at a single speed, to move the rod string up and down. Because the load changes as the rod string rises and falls, the stress or tension on the rod string varied at different points during the cycle.

A device known as a dynamometer produces a graph (dynagraph) that records the motion and forces occurring during pump operation. The dynagraph indicates polished rod load versus polished rod position. From the dynagraph, a number of phenomena can be observed.

Ideally, the sucker rod pump system would be operated to pump the maximum amount of oil without overstressing the rod system and thereby causing its premature failure.

Overtravel occurs when the pull rod stroke length is longer than the polish rod stroke length. When the pull rod stroke length is shorter than the polish rod stroke length, undertravel occurs. In either case, the rods are being stretched or compressed unnecessarily, and this is primarily caused because the natural harmonics of the rod string are not matched with the natural harmonics of the pump prime mover. Overtravel, undertravel, and fluid pound adversely affect the efficiency of pump operation and damage pump parts, causing costly downtime. Thus, there is a need to automatically adjust the speed of a pump motor to improve its efficient operation and to minimize damage to the pump rods, which not only results from fluid pound, but also from the natural loading and unloading of the rod string from the reciprocating action of the prime mover at the surface.

SUMMARY OF THE INVENTION

According to the embodiments described herein, an apparatus is disclosed for automatically controlling the speed of a pump motor comprising means for sensing an actual position of a polish rod, means for comparing the sensed actual position with a plurality of predetermined critical positions, and means for adjusting a speed of the pump motor at the plurality of predetermined critical positions to match the natural mechanical harmonics of the properly operating rod pump system.

The present invention also includes a method for automatically controlling the speed of a pump motor is disclosed comprising detecting an actual position of a polish rod in a stroke cycle, comparing the actual position with a user-selectable position, associating a speed change with the user-selectable position, and adjusting the speed of the pump motor by a user/operator rate of change at the user-selectable position when the user-selectable position is equal to the actual position.

Advantages and other features of the invention will become apparent from the following description, the drawings, and the claims.

DESCRIPTION OF THE DRAWING

FIG. 1 is a block diagram of a Rod Saver motor speed controller according to one embodiment of the invention;

FIG. 2 is a diagram of a pump including rod position detection means according to one embodiment of the invention;

FIG. 3 is a graph showing the pump speed change in relation to the position of the pump rod according to one embodiment of the invention;

FIG. 4 is a graph showing the position of the pump rod both before and after the Rod Saver motor speed controller is invoked according to one embodiment of the invention;

FIGS. 5A-5C are schematic diagrams of the Rod Saver motor speed controller according to one embodiment of the invention;

FIG. 6 is a graph showing the pump stroke position and the motor speed when the Rod Saver motor speed controller is not operated according to one embodiment of the invention;

FIG. 7 is a graph showing the pump stroke position, the change in pump motor speed, and the motor torque according to one embodiment of the invention;

FIG. 8 is a graph showing the pump stroke position, the change in pump motor speed, and the pump rod tension according to one embodiment of the invention;

FIG. 9 is a second graph showing the pump stroke position, the change in pump motor speed, and the pump rod tension according to one embodiment of the invention; and

FIG. 10 is a graph showing the pump stroke position, the torque of the motor, and the pump rod tension according to one embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

In accordance with the embodiments described herein, a Rod Saver motor speed controller and method are disclosed which identifies the position of a pump rod and, at user-selectable critical positions of the rod, adjusts the speed of the motor to change the movement of the pump rod. A variable frequency electric motor is used and the speed can be adjusted up or down. However, this speed control technique can be adapted for use with air-balance systems to change the speed of reciprocation of the pump rod (by varying the air pressure in such systems), or through other mechanical means.

In the following paragraphs and in the figures, reference is made to a variable frequency motor. The variable frequency motor may be any type of motor, including, but not limited to, an electrical, a mechanical, or a hydraulic motor. Further, the invention may be practiced using any means of changing the speed of the pump rod, including changes in gear ratios, air pressure changes, in air ballast units, variable frequency drive motors, direct current drive motors, and so on, without departing from the spirit of the invention.

In FIG. 1, according to one embodiment, a block diagram depicts a Rod Saver motor speed controller 100.

The schematic representation shows that the Rod Saver motor speed controller 100 receives both a pump rod position 40, one or more user-selected critical positions 50 and user defined rate of change 90 of the speed adjustment. With this information, the Rod Saver motor speed controller 100 adjusts a pump rod motor 30.

The pump rod position information 40 can be gathered in a number of ways, familiar to those of ordinary skill in the art. Sensors may be positioned on the pump components, for example, as depicted in FIG. 2. In one embodiment, the pump 60 includes an inclinometer 12 positioned atop a walking beam of the pump. The walking beam causes the pump rod 14 to move upwards and downwards, according to well-known principles. By reading the inclinometer 12, the Rod Saver motor speed controller 100 can ascertain a position of the pump rod 14. Any means of detecting the relative position of the polish rod can be substituted in the present Rod Saver without departing from the spirit of the invention.

Other sensors can also be situated upon the pump 60 in various locations, to provide additional information about the rod position, such as a switch contact closure devices, or other information such as a fluid meters or a motor torque meters, again without departing from the scope of the present Rod Saver invention.

Alternatively, the pump rod position 40 can be calculated, based upon the minimum and maximum movement of the pump rod 14 and the speed of the pump motor 30. Since the pump rod 14 moves in an upward and downward fashion, a graph of the pump rod position 40 is sinusoidal. The Rod Saver motor speed controller 100 can use any or all of these methods to ascertain the pump rod position 40.

Returning to FIG. 1, the Rod Saver motor speed controller 100 further includes an input device 70, according to one embodiment. The input device 70 is any device that enables the user-selected critical position(s) 50 to be entered by a user, such as a pump operator. The input device can be a keypad or other keyboard, a touch screen display, or a personal computer, as examples. The critical positions 50 are positions of the pump rod 14 which are selected based upon empirical observation, known pump rod behavior, or other criteria. The user-selected critical positions 50 are triggering points that cause the Rod Saver motor speed controller 100 to adjust the speed of the pump motor 30 by user/operator defined speed adjustments 90.

In one embodiment, the Rod Saver motor speed controller 100 also receives user-selected rate of speed change data 80 from the user. Each critical position 50 can be identified with a different amount of speed adjustment 90. The rate of change 80 specifies the rate at which the motor speed changes for a given critical position 50.

The position of the pump rod, also known as the stroke position, is described graphically as a sinusoid, as depicted in FIG. 3, according to one embodiment. The parts of the sinusoid depicting both the downstroke and the upstroke of the pump rod 14 are indicated.

Three user-defined critical positions 50 are also shown, as indicated by stars along the graph. Although three time positions, t_(d1), t_(d2), and t_(u), are depicted, the invention can be practiced using any number of critical positions. Vertical lines are shown running through the critical positions 50. The Rod Saver motor speed controller 100 uses the critical positions 50 to adjust the speed of the pump motor 30.

Also featured in the graph is a thicker line, representing a change in speed of the pump motor 30, shown as “pump speed change.” During the downstroke, at a time, t_(s), prior to the first critical position, t_(d1), the motor speed is increased, as shown. Once the target motor speed change is reached, the motor speed change remains constant until reaching the second critical position motor speed change at t_(d2).

In one embodiment, the motor speed continues to drop until reaching motor speed position, at t_(v). Prior to reaching the bottom of the stroke, the motor speed stops decreasing and no motor speed change is initiated until motor speed position, at t_(w), which is subsequent to the beginning of the upstroke.

During the upstroke, a single critical position is defined, at time position, t_(u), according to one embodiment. From time position, t_(w), until the user-selected critical position, t_(u), the motor speed steadily accelerates at a constant rate. After time t_(u), the rate of speed change again decreases, then returns to its original speed.

The effect of the motor speed level adjustment is evident in FIG. 4, according to one embodiment. Both the original stroke position and the adjusted stroke position are shown. Between the first critical position (t_(d1)) and the third critical position (t_(u)), the bend of the sinusoidal curve for the adjusted stroke position is both wider and flatter. In the sinusoidal representation, it is evident that the rod speed slows through the bottom of the downstroke for a longer period of time before returning to 100% motor speed position.

As FIG. 1 illustrated, the Rod Saver motor speed controller 100 obtains user-selected critical positions 50 and the pump rod position 40 and adjusts the speed of the pump motor 30 accordingly. In one embodiment, the pump rod position is continuously being adjusted at the user-selected critical positions. Ideally, this adjustment makes the pump 60 operate more efficiently by increasing the amount of oil pumped per unit of time and by minimizing rod compression or stretching, thereby increasing the mean time between failures of the pump rod system.

The Rod Saver motor speed controller 100 performs these adjustments using discrete logic, software, or a combination of the two. One embodiment is depicted in the schematic diagrams of FIGS. 5A-5C. As is shown in FIG. 5A, the actual stroke position (STRKEPOS) is compared to each of the three user-selected critical positions (UPSPDPOS, DNSP1POS, and DNSP2POS).

During the upstroke, the upstroke critical position (at t_(u) in FIG. 3) value (UPSPDPOS) is compared to actual rod position, its output is set active (as long as other permissives are active) when they are approximately equal and fed to activate switch 42 (FIG. 5B). The output of the switch 42 is fed into a second switch 44. The second switch 44 is active only when the first downstroke critical position (at t_(d1) in FIG. 3) value (DNSPLPOS) is approximately equal to actual rod position. Then, the downspeed (DNSP1ADJ) value is outputted; otherwise, the output of switch 42 is outputted. Likewise, the output of the second switch 44 is fed into a third switch 46. The third switch 46 is active only when the second downstroke critical position (at t_(d2) in FIG. 3) value (DNSP2POS) is approximately equal to actual rod position. Then, the downspeed 2 adjustment (DNSP2ADJ) value is outputted; otherwise the output of switch 44 is outputted.

In FIG. 6, a graph depicts the pump stroke position (thin line) and the motor speed (fat line) when the Rod Saver motor speed controller 100 is not operated. As expected, the pump stroke position is a sinusoid. Notice that the speed does not change during the downstroke of the pump rod 14. Also, during the upstroke, the speed droops at point, t_(L), due to a sharp increase in load, that is, the lifting of the oil and not a speed adjustment.

In FIG. 7, a graph depicts the pump stroke position (medium line). This time, however, the Rod Saver motor speed controller 100 is activated. Both the motor speed change (fat line) and the motor torque (thin line) are also represented. The torque is negative because the Rod Saver motor speed controller 100 is decreasing the speed of the pump motor during the downstroke of the pump rod 14.

In FIG. 8, a graph similarly shows the pump stroke position (medium line) and the motor speed change (fat line), as in FIG. 7. However, instead of motor torque, the graph shows pump rod tension (thin line). The rod tension is minimal at the top of the stroke position, but increases as the downstroke is almost complete. About halfway through the upstroke, the rod tension begins to decrease again.

As in FIG. 8, the graph of FIG. 9 shows the pump stroke position (medium line), the change in motor speed (fat line), and the pump rod tension (thin line). The slope of the downstroke is not a mirror image of the slope of the upstroke. Although the change in motor speed line is different from that in FIG. 8, the pump rod tension is similar to the previous graph.

In FIG. 10, a graph depicts the familiar sinusoidal pump stroke position (medium line). Instead of showing motor speed (FIG. 6) or change in motor speed (FIGS. 7-9), the graph shows the pump rod tension and the torque of the motor.

While the invention has been described with respect to a limited number of embodiments, those skilled in the art will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of the invention. 

1. A Rod Saver apparatus for automatically controlling the speed of a pump rod throughout its normal operating cycle comprising: means for determining an actual position of said pump rod; means for comparing the sensed actual position with a plurality of predetermined critical positions; and means for adjusting said pump rod speed at the plurality of predetermined critical positions during each cycle of said pump rod.
 2. The Rod Saver apparatus of claim 1 wherein a pump motor moves said pump rod and said means for adjusting said pump rod speed adjusts the speed of said pump motor.
 3. The Rod Saver apparatus of claim 2 wherein said pump motor is a variable frequency motor.
 4. The Rod Saver apparatus of claim 3 wherein said variable frequency motor is selected from an electrical, a mechanical, or a hydraulic motor.
 5. The Rod Saver apparatus of claim 1 wherein plurality of predetermined critical positions are selected by a user of said Rod Saver apparatus.
 6. The Rod Saver apparatus of claim 1 wherein said means for determining said pump rod actual position is a means for sensing said pump rod actual position.
 7. The Rod Saver apparatus of claim 6 wherein said means for sensing said pump rod actual position is a sensor situated on a pump attached to said pump rod.
 8. The Rod Saver apparatus of claim 2 wherein said means for determining said pump rod actual position is determined from the minimum and maximum movement of said pump rod and the speed of said pump motor.
 9. A Rod Saver method for automatically controlling the speed of a pump rod throughout its normal operating cycle comprising: determining an actual position of said pump rod in a stroke cycle; comparing the actual position with a user-selectable position; associating a change of speed with the user-selectable position; and adjusting said pump rod speed by the rate of change at the user-selectable position when the user-selectable position is equal to the actual position.
 10. The Rod Saver method of claim 9 wherein a pump motor moves said pump rod and said pump rod speed is adjusted by adjusting the speed of said pump motor.
 11. The Rod Saver method of claim 10 wherein said pump motor is a variable frequency motor.
 12. The Rod Saver method of claim 12 wherein said variable frequency motor is selected from an electrical, a mechanical, or a hydraulic motor.
 13. The Rod Saver method of claim 9 wherein said pump rod actual position is determined by detecting said actual position.
 14. The Rod Saver method of claim 13 wherein said pump rod actual position is detected by a sensor situated on a pump attached to said pump rod.
 15. The Rod Saver method of claim 10 wherein said pump rod actual position is determined from the minimum and maximum movement of said pump rod and the speed of said pump motor. 